1. Field of the Invention
The present invention relates an apparatus and method for making carbon fibers.
2. Invention Background
The present invention relates an apparatus and method for making carbon fibers.
Carbon fibers are known to be produced by the two-stage pyrolysis of rayon, polyacrlonitrile (PAN) or petroleum (or coal) pitch precursor fibers. Other synthetic fibers that have been considered as possible precursors for carbon fibers include aromatic polyamides, polyvinyl alcohol, polyphenylenes, polyvinyl chloride and polyoxadiazoles.
Generally, production of carbon fibers has been carried out by first heat treating precursor (raw) fibers in an oxidizing environment. Tension can be applied to the fibers during this heat treatment to retard fiber shrinkage and to maintain molecular orientation. This step is usually carried out at about 191 to 279 degrees Celsius for about one half hour to several hours. This step, known as stabilization forms chemical bonds that resist burning and increase the flash point of the fibers. Once the fiber is stabilized, it is further processed by carbonization through further heat treating in a non-oxidizing environment. Usually, the carbonization takes place at temperatures in excess of 525 degrees Celsius and in a nitrogen atmosphere.
The resultant carbon fibers are primarily fibers having in excess of 92 percent carbon. Higher carbonization temperatures can be used and can result in complete or nearly complete graphitization of the fibers. Fibers in excess of 99 percent carbon are known to be produced through this process.
U.S. Pat. No. 5,700,573 teaches a biregional carbon fiber and method of making them. The '573 patent shows a fiber that, instead of being completely carbonized, has an outer carbonized sheath surrounding an inner noncarbonized core. The biregional fiber is produced from a homogeneous polymeric material in which an outer fiber portion of the polymeric material is oxidation stabilized and then carbonized to form two distinct regions in the fiber. A preferred polymeric material for this purpose is a standard acrylic polymer (i.e. copolymers and terpolymers of acrylonitrile, in which the copolymers and terpolymers contain at least 85 mole percent acrylic units and up to 15 mole percent of one or more vinyl monomers copolymerized therewith or optionally a subacrylic polymer).
Current production techniques call for batch carbon fiber formation. Therefore, fibers are maintained at an oxidizing temperature in the presence of oxygen for a length of time and then transferred to a non-oxidizing environment, such as an oxygen-free tube furnace, for carbonization or graphitization. As a result, batch carbon fiber production is time consuming.
In addition, U.S. Pat. No. RE34,162 to Boyd, Jr. teaches the continuous carbonization of previously stabilized fibers with the use of a known continuous line carbonizer.
None of the prior art production techniques employ a continuous process for continuously carbonizing the precursor fiber.